The H.323 Protocol Specification

The H.323 protocol suite allows dissimilar communication devices to communicate with each other. H.323 (which is implemented primarily at versions 4 and 5 as of the time of this writing) is a sometimes Byzantine international protocol published by the ITU that supports interoperability between differing vendor implementations of telephony and multimedia products across IP-based networks. H.323 entities provide for real-time audio, video, and/or data communications. Support for audio is mandatory; support for data and video is optional.

The H.323 specification defines four different H.323 entities as the functional units of a complete H.323 network (see Figure 1). These components of an H.323 system include endpoints (terminals), gateways, gatekeepers, and multipoint control units (MCUs).

 

Figure 1: H.323 Entities

Endpoints (telephones, softphones, IVRs, voice mail, video cameras, etc.) are typically devices that end-users interact with. MS Netmeeting is an example of an H.323 endpoint. Endpoints provide voice-only and/or multimedia such as video and real-time application collaboration.

Gateways handle signaling and media transport, and are optional components. Gateways typically serve as the interface to other types of networks such as ISDN, PSTN, or other H.323 systems. You can think of a gateway as providing “translation” functions. For example, an H.323 gateway will handle conversion of H.323 to SIP or H.323 to ISUP (ISUP (ISDN User Part) defines the interexchange signaling procedures for the trunk call control). Another way to think of this is that a gateway provides the interface between a packet-based network (e.g., a VoIP network) and a circuit-switched network (e.g., the PSTN). If a gatekeeper exists, VoIP gateways register with the gatekeeper and the gatekeeper finds the “best” gateway for a particular session.

Gatekeepers, which are also optional, handle address resolution and admission to the H.323 network. Its most important function is address translation between symbolic alias addresses and IP addresses. For example, in the presence of a gatekeeper, it is possible to call “Tom,” rather than 192.168.10.10. Gatekeepers also manage end points’ access to services, network resources, and optionally can provide additional services. They also monitor service usage and provide limited network bandwidth management. A gatekeeper is not required in an H.323 system. However, if a gatekeeper is present, terminals must make use of the services offered by gatekeepers. RAS defines these as address translation, admissions control, bandwidth control, and zone management. The gatekeeper and gateway functionalities are often present on a single physical device.

MCUs support multiparty conferencing between three or more endpoints. The H.323 standard allows for a variety of ad hoc conferencing scenarios, either centralized or decentralized.

Back-end servers (BES) are an important supplementary function in an H.323-based environment. BES may provide services for user authentication, service authorization, accounting, charging and billing, and other services. In a simple network, the gatekeeper or gateway provides such services.